Players start twitching violently after moving. They continue moving in-game, but my game renders them as still in that same spot. This happens to mobs aswell. I am using Badlion client in the video, but this happens in modded Minecraft and the normal client.
The server stopped sending updates to the client, or the client is for some reason unable to interpret the packet from the server.
The last packet the client received said that the player you see on screen had some velocity so it predicts where the player will be in the future, but future packets were never received, so when the next tick occurs it moves back to it's previous position visually.
I've seen this issue when creating my minecraft mods if I don't read/write a packet correctly. There is nothing you as the end user can do about it, since you say it occurs on vanilla. This means the server (hypixel) is not sending entity update packets for that particular player to your client.